Introduction to Fourier Transform Infrared Spectroscopy

1. 张远征 13601358075 01068474806-669 Zhang.yuanzheng@brukeroptics.cn

2. 傅立叶红外光谱介绍

3. 电磁波 Energy [eV] Wavenumber [cm-1] Wavelength [m] Short Wave Radio Waves Micro Wave Gamma Ray X-Ray UV Visible Infrared

4. 光与分子的作用 分子激发产生振动

5. 振动的种类? 例如: 水 伸缩振动 变形振动 不对称伸缩振动 对称伸缩振动

6. 100 95 90 85 Transmission [%] 80 75 70 水的红外图 65 60 3500 3000 2500 2000 1500 wavenumber cm-1

7. 正己烷 50多不同的振动

8. 100 80 60 Transmission [%] 40 C-H stretch C-H deformation 20 4000 3500 3000 2500 2000 1500 1000 wavenumber cm-1 正己烷 „指纹区“

9. 红外光谱分为三个范围: NIR MIR FIR 15.000 cm-1 4.000 cm-1 400 cm-1 5 cm-1

10. 如何得到一张图 色散型红外光谱仪 傅立叶变换红外光谱仪

11. 色散型红外光谱仪 Detector Detector 缺点:- 速度慢- 光通量低 => 灵敏度低 (S/N ratio) 优点:- 不需要计算机

12. 定镜 L 动镜 光源 L + x 分束器 x=0 傅利叶变换红外光谱仪原理 x

13. fixed mirror M1 source Beam splitter x=0 例 1: x =0, 相长干涉  1. Beam part (定镜) L x 2. Beam part (动镜) L + x 结果 Detector

14. fixed mirror M1 source Beam splitter x=1/2 Detector 例 2: x =1/2, 相消性干涉  1. Beam part (定镜) L x 2. Beam part (动镜) L + x 结果 0

15. fixed mirror M1 source Beam splitter x= Detector example 3: x = , constructive Interference  1. Beam part (fixed) L x 2. Beam part (movable) L + x Resulting signal 0

16. fixed mirror M1 source Beam splitter x=3/2 Detector example 4: x =3/2, destructive Interference  1. Beam part (fixed) L x 2. Beam part (movable) L + x Resulting signal 0

17. 光源 Intensity Frequence 单色光源 监测器信号 Intensity Mirror motion 单色光源的调制信号

18. Resulting detector signal Intensity Mirror motion 总和: Intensity Mirror motion Entstehung des Interferogramms 9条单一频率的光源 Intensity Frequence

19. 干涉图的来源 红外光源 检测器信号 Intensity Intensity Frequency X, moving mirror

20. 透射光谱 1.) In the empty sample compartment an Interferogram is detected. The result of the FOURIER transformation is R(ν). Detector intensity X, moving mirror Fourier-Transformation 0.40 0.30 Single channel intensity 0.20 0.10 4000 3500 3000 2500 2000 1500 1000 500 wavenumber cm-1

21. 透射光谱 2.) A second interferogram is detected with the sample placed in the sample compartment. The result of the FOURIER transformation is S(ν). S(ν) shows similarities to the reference spectrum R(v), but has lower intensities at the regions the sample absorbs radiation. Detector intensity X, moving mirror Fourier-Transformation 0.40 0.30 Single channel intensity 0.20 0.10 4000 3500 3000 2500 2000 1500 1000 500 wavenumber cm-1

22. 0.40 0.30 Single channel intensity 0.20 0.10 4000 3500 3000 2500 2000 1500 1000 500 wavenumber cm-1 ratio 100 80 60 Transmission [%] 40 20 4000 3500 3000 2500 2000 1500 1000 500 wavenumber cm-1 透射光谱 The transmission spectrum T(ν) is calculated as the ratio of the sample and reference single channel spectra: T(ν) = S(ν)/R(ν).

23. 100 80 1.0 60 Transmittance [%] 0.8 40 0.6 20 Absorbance Units 0.4 0 6000 5000 4000 3000 2000 1000 Wavenumber cm-1 0.2 0.0 6000 5000 4000 3000 2000 1000 Wavenumber cm-1 Absorbance <-> Transmission - Why? Transmission Absorbance Lambert-Beer‘s law: AB = -log (S(ν)/R(ν)) AB =  • c • b T(ν) = S(ν)/R(ν)

24. Fixed mirror L Moving mirror Source L + x Beamsplitter x=0 Principle layout of FT-IR spectrometer x

25. Layout of an FT-IR spectrometer (TENSOR series) Electronic Source compartment Detector Aperture wheel Filter wheel Sample compartment Sample position Interferometer compartment

26. Differences between NIR, MIR, FIR Optical components: NIR: Source : tungsten lamp Optical material : Quartz Detector: Ge, InGaAs MIR: Source: Globar Optical material: KBr, ZnSe Detector: DTGS, MCT FIR: Source : Globar, Hg lamp Optical material : PE, CsI Detector: DTGS, Bolometer

27. Fourier Transformation (FT)

28. ~ ~ ~    The Fourier Transform Data acquisition results in a digitized interferogram, I(x), which is converted into a spectrum by means of the mathematical operation called a Fourier Transform (FT). The general equation for the Fourier Transform is applicable to a continuous signal. If the signal (interferogram) is digitized, however, and consists of N discrete, equidistant points, then the discrete version of the FT (DFT) must be used: S(k . Δ ) = Σ I(n  Δx)  exp (i2πk  n/N) The continuous variables x and have been replaced with n Dx and k D , representing the n discrete interferogram points and the k discrete spectrum points. The fact that we now have a discrete, rather than continuous, function, and that it is only calculated for a limited range of n (i.e. the measured interferogram has a finite length) leads to important effects known as the picket-fence effect and leakage.

29. fixed mirror M1 L movable mirror M2 source L + x Beam splitter x=0 detector 高光谱分辨 x 低光谱分辨

30. 0.55 0.55 0.50 0.50 Single channel 0.45 0.45 Single channel 0.40 0.40 0.35 0.35 1,808 1,806 1,804 1,802 1,800 1,798 1,796 1,808 1,806 1,804 1,802 1,800 1,798 1,796 Wavenumber, cm-1 Wavenumber, cm-1 ~ ~   添零 The picket-fence effect occurs if the interferogram contains frequency components which do not exactly coincide with the data point positions, k.Δ , in the spectrum. The effect can be thought of as viewing the spectrum through a picket fence, thereby hiding those frequencies that are behind the pickets, i.e. between the data point positions k.Δ . In the worst case, if a frequency component is exactly between two sampling positions, a signal reduction of 36% can occur. The picket-fence effect can be reduced by adding zeros to the end of the interferogram (zero filling) before the DFT is performed. This interpolates the spectrum, increasing the number of points per wavenumber. The increased number of frequency sampling positions reduces the error caused by the picket-fence effect. Generally, the original interferogram size should always be at least doubled by zero filling, i.e. zero filling factor (ZFF) of two is chosen. Zero-filling interpolates using the instrument line-shape, and in most cases is therefore superior to polynominal or spline interpolation methods that are applied in the spectral domain. Zero-filling factor 2 Zero-filling factor 8

31. ~ ~ ~    截趾函数 A BOXCAR (no apodization) In a real measurement, the interferogram can only be measured for a finite distance of mirror travel. The resulting interferogram can be thought of as an infinite length interferogram multiplied by a boxcar function that is equal to 1 in the range of measurement and 0 elsewhere. This sudden truncation of the interferogram leads to a sinc( ) (i.e. sin( )/ ) instrumental lineshape. For an infinitely narrow spectral line, the peak shape is shown at the top of the figure on the right. The oscillations around the base of the peak are referred to as “ringing”, or “leakage”. The solution to the leakage problem is to truncate the interferogram less abruptly. This can be achieved by multiplying the interferogram by a function that is 1 at the centerburst and close to 0 at the end of the interferogram. This is called apodization, and the simplest such function is a ramp, or “triangular apodization”. The choice of a particular apodization function depends on the objectives of the measurement. If the maximum resolution of 0.61/L is required, then boxcar apodization (i.e no apodization) is used. If a resolution loss of 50% (compared to the maximum resolution of 0.61/L) can be tolerated, the HAPP-GENZEL or, even better, 3-Term BLACKMAN-HARRIS function is recommended. B Triangular C Trapezoidal D HAPP-GENZEL E 3-TERM BLACKMAN-HARRIS

32. Evaluation of IR spectra

33. 光谱评价 • 定性分析： • 1. 鉴定未知物 • 2. 核对已知物 • 定量分析

34. 100 80 60 Transmission [%] 40 20 4,000 3,500 3,000 2,500 2,000 1,500 1,000 500 Wavenumber / cm-1 未知物的鉴定 a) 通过光谱解析推出分子结构

35. 不同有几类分子的红外吸收 烷烃 烯烃 芳香烃 卤化物 羧酸盐 酸酐 内酯

36. 未知物的鉴定 b.) 与标准谱库比较 e.g. by using OPUS/Search

37. 验证已知物 identical material = identical IR spectrum - What you have: sample - What you need: reference library - What you do: comparison with reference library - What you get: identification

38.                            Reference library structure 1.) Measure reference sample 2.) Calculate average spectrum & threshold values 3.) Library structure & validation Absorbance Absorbance Wavenumber / cm-1 Wavenumber / cm-1

39. Identified sample: material X Identifying new samples 1.) Measure new samples 3.) Identify material 2.) Compare with library

40. Calibration 4 3 Absorbance X 2 1 Wavelength Analysis 4 3 Absorbance X 2 1 Concentration Quantitative evaluation of spectra • - What you have: • sample • - What you need: • calibration set • - What you do: • comparison with calibration set • - What you get: • concentration value • There are two different forms of calibration: • Univariate calibration (OPUS) • - Correlates just one piece of spectral information (e.g. peak height or peak area) with the reference values of the calibration set. • Multivariate calibration (OPUS/QUANT) • - Correlates considerably more spectral information • - higher degree of precision • - reduced chance of error • OPUS/QUANT uses the Partial Least Squares (PLS) Method.

41. 4 3 Absorbance 2 1 Concentration Setup of a Quant Method 1.) Measure calibration spectra 2.) Build calibration set (Quant Method) 3.) Validate calibration set Absorbance Wavenumber / cm-1

42. Concentration:58 vol. % Determine quantitative results (e.g. concentration values) 1.) Measure sample 3.) Result 2.) Compare with calibration set

43. FT-IR measurements

44. Enter sample name Start the background and sample measurement

46. Sampling bandwidth in interferogram domain

47. The FT of a measured interferogram yields a complex spectrum. The aim of the phase correction is to calculate the real spectrum. Interpolation of the spectrum by adding zeros to the end of the interferogram

48. Defines the separation of adjacent peaks

49. Acquisition mode single sided double sided fast backward forward and backward

50. If you have any further questions about IR spectroscopy, please contact the application team of Bruker Optics: www.brukeroptics.com North America: Bruker Optics Inc 19 Fortune Drive Billerica, MA 01821, USA Phone: +1 978 439 9899 Fax: +1 978 663 9177 info@brukeroptics.com Asia: Bruker Optik Asia Pacific Ltd. Unit 601, 6/F, Tower 1 Enterprise Square No. 9, Sheung Yuet Road, Hong Kong Phone: +852 27966100 Fax: +852 4927966109 asiapacific@brukeroptics.com.hk Europe: Bruker Optik GmbH Rudolf-Plank-Str. 27 76275 Ettlingen, Germany Phone: +49 7243 504 600 Fax: +49 7243 504 698 info@brukeroptics.de